US 7325716 B2
Apparatus and methods of fabricating a bump limiting metallization structure including a two-step bump reflow process that reduces intermetallic compound porosity, increases bump strength, improve die yield, and device reliability. The first step comprises annealing a bump limiting metallurgy and a solder plug at a temperature below the liquidus temperature of the solder plug to form a dense intermetallic compound layer between the solder plug and the bump limiting metallurgy. The second step comprises heating the bump limiting metallurgy and the solder plug at a temperature above the liquidus temperature of the solder plug to form a solder bump.
1. An apparatus, comprising:
a solder bump;
a wetting layer; and
a dense intermetallic compound, having a density of greater than about 0.95, formed between said solder bump and said wetting layer.
2. The apparatus of
3. The apparatus of
4. The apparatus of
5. The apparatus of
6. The apparatus of
1. Field of the Invention
Embodiments of the present invention relate to microelectronic device fabrication. In particular, an embodiment of the present invention relates to methods of fabricating bump limiting metallurgy (BLM) structure, which results in a dense intermetallic compound layer for increased bump strength and adhesion to metal pad and passivation layer.
2. State of the Art
The microelectronic device industry continues to see tremendous advances in technologies that permit increased circuit density and complexity, and equally dramatic decreases in power consumption and package sizes. Present semiconductor technology now permits single-chip microprocessors with many millions of transistors, operating at speeds of tens (or even hundreds) of MIPS (millions of instructions per second), to be packaged in relatively small, air-cooled microelectronic device packages. A result of such high density and high functionality in microelectronic devices has been the demand for increased numbers of external electrical connections to be present on the exterior of the microelectronic die in order to connect the microelectronic die to other components, such as an interposer.
The connection mechanism for such high density connection is generally wafer level bumping through a C4 (controlled collapse chip connection) process, because the size of the balls or bumps of the array can be made smaller to provide a higher density thereof, and thereby creating a greater number of connections from microelectronic die. A C4 is formed by placing an amount of solder on a microelectronic die pad and heating the solder above its melting point. The surface tension associated with the liquid solder causes the solder to form a solder ball. The solder ball retains its shape as it cools to form a solid solder ball or bump. While the description here is for wafer level bumping, the principle could be applied to ball grid array packaging.
As shown in
As shown in
The BLM 406 provides a reliable electrical and mechanical interface between the bond pad 424 and the solder bump 416. A typical BLM includes at least one conductive layer which will act to adhere to the solder ball 416 and bond pad 424 and to prevent contamination between the solder bump 416 and the microelectronic die 402. Of course, multiple layers may be used to form the BLM 406. For example, the BLM 406 may consist of an adhesion layer 442 for attachment to the bond pad 424, a barrier layer 444 over the adhesion layer 442 to prevent contamination between the solder bump 416 and microelectronic die (not shown), and a wetting layer 446 between the barrier layer 444 and the solder bump 416 to “wet” or adhere to the solder bump 416 material.
A nickel-vanadium-nitrogen alloy may be used as at least one layer in the BLM 406, particularly as an wetting layer 446. The solder bump 416 generally contains tin, such as lead/tin alloys or lead free solders, such as substantially pure tin or high tin content alloys (90% or more tin), such as tin/bismuth, eutectic tin/silver, ternary tin/silver/copper, eutectic tin/copper, and the like. However, solder bump 416 reflow attachment discussed above generally comprises a one-step reflow process. This one-step reflow process generates voids 138, shown in
Therefore, it would be advantageous to develop apparatus and techniques to form an intermellatic compound layer which would have improved adhesion and device reliability.
While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings to which:
In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention, although different, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described herein, in connection with one embodiment, may be implemented within other embodiments without departing from the spirit and scope of the invention. In addition, it is to be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, appropriately interpreted, along with the full range of equivalents to which the claims are entitled. In the drawings, like numerals refer to the same or similar functionality throughout the several views.
An embodiment of the present invention comprises a two-step bump reflow process that reduces intermetallic compound porosity, increases bump strength, improve die yield, and device reliability.
The conductive pad 102 is connected to a conductive trace 108 through a conductive via 112. The conductive trace 108 is routed to the microelectronic die (not shown), as will be understood by those skilled in the art. A passivation layer 114, such as polyimide, may be deposited on the interconnect structure 104 and patterned to expose at least a portion of the conductive pad 102.
The BLM structure 120 is fabricated to contact the conductive pad 102 and may be formed with a first adhesion layer 122, a barrier layer 124, a second adhesion layer 126, and a wetting layer 128. The first adhesion layer 122 may be formed on a portion of the passivation layer 114 and the conductive pad 102. The first adhesion layer 122 is selected to adhere well to the conductive pad 102 and the passivation layer 114, and may include, but is not limited to, titanium and alloys thereof. The barrier layer 124 is formed on the first adhesion layer 122 to limit the diffusion of material(s) within a solder bump to be formed on the BLM structure 120 to the adhesion layer 122, conductive pad 102, and interconnect structure 104 and may include, but is not limited to, comprises aluminum, aluminum alloys, molybdenum, molybdenum alloys, and the like. The second adhesion layer 126 is formed on the barrier layer 124 and selected provide adhesion between the barrier layer 124 and the wetting layer 128. The second adhesion layer 126 may include, but is not limited to, titanium and alloys thereof. The wetting layer 128 is formed on the second adhesion layer 126 to provide an easily wettable surface for the molten solder bump during assembly for good bonding of the solder to the barrier layer 124 and may include, but is not limited to, nickel-containing alloys, such nickel-vanadium-nitrogen alloy. The first adhesion layer 122, the barrier layer 124, the second adhesion layer 126, and the wetting layer 128 may be formed by any method known in the art, including but not limited to, deposition by physical vapor deposition (sputtering), chemical vapor deposition, evaporation, electroplating, electroless plating, and ion beam deposition.
A solder plug 130 is formed on the wetting layer 128 and may comprise a tin-containing material, including but not limited to, lead/tin alloy, and substantially pure tin or high tin content alloys, such as such as tin/bismuth, tin/silver, ternary tin/silver/copper, tin/copper, and the like. The solder plug 130 may be fabricated by any known method in the art including, but not limited to, electroplating, jet plating, and screen printing of a solder paste.
It is, of course, understood that any number of layers may comprise the BLM structure 120. For example, if a selected material can act as a barrier layer and a wetting layer, and is sufficiently adhesive to the solder plug 130 and the conductive pad 102, then only one material layer would be required to form the BLM structure 120.
The solder plug 130 is subsequently reflowed with a two-stage process. A first stage comprises a solid state diffusion by annealing the solder plug 130 and the BLM structure 120 at a temperature lower than the melting temperature of the solder plug 130. The annealing stage results in the formation of a dense intermetallic compound layer 132 from the diffusion of at least one metal of the wetting layer 128 into the solder plug 130. The annealing stage can be conducted at a temperature within about 20 percent of the liquidus temperature of the solder plug 130. The temperature for the solid state diffusion needs to be chosen to effectively diffuse at least one component in the wetting layer 128 and form the intermetallic compound, but excessive heat in this stage will adversely promote voids in the BLM structure.
The second stage is a reflow stage, preferably a low thermal budget bump reflow, in which the solder plug 130 is reflowed to form a sphere or solder bump 134 without excessive intermetallic growth due to the dense intermetallic compound layer 132 formed in the annealing stage, as shown in
In one embodiment of the present invention, the BLM structure 120 is fabricated with a first adhesion layer 122 comprising a titanium layer about 1000 angstroms thick contacting the conductive pad 102, a barrier layer 124 comprising aluminum layer about 10,000 angstroms thick abutting the first adhesion layer 122, a second adhesion layer 126 comprising a titanium layer about 1000 angstroms thick abutting the barrier layer 124, and a wetting layer 128 comprising a nickel-vanadium-nitrogen alloy layer about 4000 angstrom thick abutting the second adhesion layer. In this embodiment, the nickel-vanadium-nitrogen alloy layer comprises about 89% nickel, about 7% vanadium, and about 4% nitrogen by weight.
A solder plug 130 comprising a lead/tin alloy (97% lead/3% tin in this embodiment) is formed on the wetting layer 128. As illustrated in the graph in
This process results in solder bump(s) without excessive intermetallic growth and without void in BLM structure 120 due to the dense intermetallic compound layer 132 formed in the annealing stage. The difference between a single stage reflow process and the two-stage process of the present invention is illustrated in
In one embodiment of the present invention, a density of the intermetallic compound layer is estimated or calculated from the wetting layer 128 reflow by the following equation:
Although the present invention is described in terms of applying to a microelectronic die, the present invention can apply to any bump limiting metallurgy including, but not limited to, those fabricated on an interposer, as will be understood by those skilled in the art.
The packages formed by the present invention may be used in a hand-held device 210, such as a cell phone or a personal data assistant (PDA), as shown in
The microelectronic device assemblies formed by the present invention may also be used in a computer system 310, as shown in
Having thus described in detail embodiments of the present invention, it is understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description, as many apparent variations thereof are possible without departing from the spirit or scope thereof.